MATHS CHALLENGE 2010
SOLUTIONS – PART 1
EASY 1
If five robots can make five new robots in five hours, how long would it take a
hundred robots to make a hundred new robots?
ANSWER: In five hours, each of the five robots makes one robot. That is, a single
robot takes five hours to make a robot. So, in five hours a hundred robots can make
a hundred robots.
EASY 2
Two poker chips are pictured below. The two visible sides add up to 10. Flipping the
chips in all possible ways, you get the sums 7, 8, 9, and 10. What are the numbers on
the other side of the chips?
ANSWER: One of the sums is 9, which gives us three scenarios:
a) The green 6 is part of the 9. Then the hidden side of the red chip is 3, and it
follows that the hidden side of the green chip is 4.
b) The red 4 is part of the 9. Then the hidden side of the green chip is 5, and it
follows that the hidden side of the red chip is 2.
c) The sum of the two hidden sides is 9.
In fact, the third scenario is impossible. The two remaining sums are 8 and 7, which
means the hidden side of the red chip would be at most 2, and the hidden side of the
green chip would be at most 4. So, the two hidden sides cannot add to 9.
EASY 3
A drawer full of socks contains 5 black socks and 6 blues socks. You take socks out
of the drawer without looking. How many socks must you take out to be sure of
having a matching pair? What if you want to be sure of having a black pair?
ANSWER: Three socks must give a matching pair, since either two of the socks are
blue, or two are black. Eight socks are needed to ensure a black pair, since the first
six you choose might all be blue.
EASY 4
The triangle of coins below is pointing upwards. By moving only three coins, arrange
for the triangle to point downwards.
ANSWER: Keep the hexagon in the middle, and move the three corner coins.
EASY 5
Three-dimensional Homer stumbles across the equation 178212 +184112 =192212. Is
the equation true or false? (Put that calculator down!)
ANSWER: The left side of the equation is EVEN + ODD = ODD, and the right side
of the equation is EVEN. So, the equation must be false.
EASY 6
Which is larger, 1000/3000 or 1001/3001? (Put that calculator down!)
ANSWER: Since 1000/3000 = 1/3 = 1001/3003, the fraction 1001/3001 must be
larger.
EASY 7
Julie takes a third of the jellybeans from a large jar. Then Tony takes a third of the
remaining jellybeans from the jar. Finally, Bob takes a third of the remaining
jellybeans, leaving 40 jellybeans in the jar. How many jellybeans were there to begin
with?
ANSWER: Bob left 40 beans, which was 2/3 of the beans. So, at that stage there
were 60 beans. That means 60 beans amounted to 2/3 of the beans when Tony took
his beans, so there must have been 90 beans at that stage. Finally, those 90 beans
were 2/3 of the beans when Julie took hers, so there must have initially been 135
beans.
EASY 8
A bat and a ball together cost $110. The bat costs $100 more than the ball. How
much does the ball cost?
ANSWER: The ball cost $5 and the bat cost $105.
EASY 9
Three tennis balls fit exactly inside a cylindrical can. Which is greater, the
circumference of the can or the length of the can? Which is greater, the combined
area of the three balls, or the area of the cylinder (excluding the base and the lid)?
ANSWER: Call the radius of the balls R. So, the length of the can is 6R and the
circumference of the can (equals the length of the equators of the balls) is 2πR. Since
π > 3, the circumference is greater than the length. Also, the area of the cylinder is
2πR x 6R = 12πR2. The area of each ball is 4πR2, so the combined area of the balls is
the same as the area of the cylinders.
EASY 10
Renae walks into an electronics store to purchase a $1000 television, and she has
three coupons. One coupon entitles Renae to a 50% discount, the second to a 30%
discount, and the third to a 20% discount. David the salesmen agrees to let Renae
use all three coupons at once. How much does Renae pay for the television?
ANSWER: The 50% discount coupon means that Renae would pay only $500. Then,
the 30% discount coupon means that Renae only has to pay 70% of that $500, and so
the television would cost $350. Finally, the 20% discount coupon applied to that
$350, lowers the price another $70, and so Renae only has to pay $280.
Note that it doesn’t matter in which order Renae applies the coupons: in any case
we’re multiplying $1000 by 0.5 x 0.7 x 0.2 = 0.28. Also, if Renae confronts a really
gullible salesmen, she might actually succeed in obtaining a 50% + 30% + 20% = 100%
discount!